Babesia microti (Bm) is a tick-borne intra-erythrocytic parasite endemic in NE and the upper Midwest. Although primarily a tick-borne disease, Bm has been transmitted by transfusion in at least 50 documented cases. Symptoms include fever, hemolytic anemia, and thrombocytopenia, typically arising 2–8 weeks following transfusion. In order to assess the risk of Bm transmission by blood transfusion in Connecticut (CT), we tested a repository of donor and recipient samples collected in 2004–2007.

METHODS: The repository consisted of frozen whole blood and serum samples collected generally 1, 3, 6, and 12 months after blood transfusions in a chronically transfused population, along with associated donor serum samples, collected at blood drives in CT. All recipient follow-up samples were screened for antibodies to Babesia microti by IFA, as were the initial samples of any seropositive recipient, using a 1:64 cut-off titer. If recipients tested IFA positive after being seronegative (seroconversion), corresponding donor sera were screened for Bm antibody to identify transfusion-transmission. Stored DNA from serial seroconverting recipient samples were also assessed by real-time PCR for Bm. We defined an evaluable transfusion for Bm as a platelet or RBC transfusion with at least one follow-up sample 14–180 days later. 107 recipients received evaluable transfusions. Altogether these recipients received 1920 evaluable RBC transfusions and 1634 evaluable platelet transfusions.

RESULTS: All follow-up samples were seronegative for Bm except for a single follow-up sample in a recipient with sickle cell anemia transfused with 45 RBC over 24 months. This sample was reproducibly seropositive in 2 labs with a titer of 1:64 and was PCR negative. Blood samples 6 weeks before and 11 weeks after the seropositive sample were seronegative, but PCR +. To investigate, 11 earlier recipient samples taken 5–21 months before the seroconversion were tested and all were seronegative, although 2/11 were PCR + (one strongly positive). Donor serum samples from 18/21 RBC transfused prior to the strongly PCR + recipient sample were negative for Bm. Three donor samples were not available. The recipient reported no exposure to ticks and lived in a non-endemic area of Connecticut. The patient had received 41 units of red cells in the two years before enrollment in the study. There were no clinical symptoms attributable to Bm.

CONCLUSION: This may be a case of transfusion-transmitted Babesia microti, despite our inability to identify a seropositive blood donor. However, the recipient may have acquired Bm from a tick bite or from earlier transfusions. The risk of Babesia microti transmission by transfusion in CT has thus been measured either as zero cases in 1920 RBC transfusions (95% CI 0.0 - 0.0016 per RBC) or as 1 case per 1920 RBC transfusions (0.005, CI 0.000013 - 0.0029 per RBC). A previous report (

Gerber, et al.
) directly measured the risk of transfusion transmission of Babesia microti in CT as 1 in 601 RBC (.0017). A recent risk estimate based on the prevalence of PCR positive CT donor samples is 1/1800 RBC (0.0006) (
Cable RG, et al.
.) This current study of chronically transfused recipients is consistent with these earlier estimates.

Author notes

Disclosure: No relevant conflicts of interest to declare.